Android HAL 层原理分析

/**
 * Every hardware module must have a data structure named HAL_MODULE_INFO_SYM
 * and the fields of this data structure must begin with hw_module_t
 * followed by module specific information.
 */
typedef struct hw_module_t {
    /** tag must be initialized to HARDWARE_MODULE_TAG */
    uint32_t tag;

    /**
     * The API version of the implemented module. The module owner is
     * responsible for updating the version when a module interface has
     * changed.
     *
     * The derived modules such as gralloc and audio own and manage this field.
     * The module user must interpret the version field to decide whether or
     * not to inter-operate with the supplied module implementation.
     * For example, SurfaceFlinger is responsible for making sure that
     * it knows how to manage different versions of the gralloc-module API,
     * and AudioFlinger must know how to do the same for audio-module API.
     *
     * The module API version should include a major and a minor component.
     * For example, version 1.0 could be represented as 0x0100. This format
     * implies that versions 0x0100-0x01ff are all API-compatible.
     *
     * In the future, libhardware will expose a hw_get_module_version()
     * (or equivalent) function that will take minimum/maximum supported
     * versions as arguments and would be able to reject modules with
     * versions outside of the supplied range.
     */
    uint16_t module_api_version;
#define version_major module_api_version
    /**
     * version_major/version_minor defines are supplied here for temporary
     * source code compatibility. They will be removed in the next version.
     * ALL clients must convert to the new version format.
     */

    /**
     * The API version of the HAL module interface. This is meant to
     * version the hw_module_t, hw_module_methods_t, and hw_device_t
     * structures and definitions.
     *
     * The HAL interface owns this field. Module users/implementations
     * must NOT rely on this value for version information.
     *
     * Presently, 0 is the only valid value.
     */
    uint16_t hal_api_version;
#define version_minor hal_api_version

    /** Identifier of module */
    const char *id;

    /** Name of this module */
    const char *name;

    /** Author/owner/implementor of the module */
    const char *author;

    /** Modules methods */
    struct hw_module_methods_t* methods;

    /** module's dso */
    void* dso;

#ifdef __LP64__
    uint64_t reserved[32-7];
#else
    /** padding to 128 bytes, reserved for future use */
    uint32_t reserved[32-7];
#endif

} hw_module_t;

/**
 * Every device data structure must begin with hw_device_t
 * followed by module specific public methods and attributes.
 */
typedef struct hw_device_t {
    /** tag must be initialized to HARDWARE_DEVICE_TAG */
    uint32_t tag;

    /**
     * Version of the module-specific device API. This value is used by
     * the derived-module user to manage different device implementations.
     *
     * The module user is responsible for checking the module_api_version
     * and device version fields to ensure that the user is capable of
     * communicating with the specific module implementation.
     *
     * One module can support multiple devices with different versions. This
     * can be useful when a device interface changes in an incompatible way
     * but it is still necessary to support older implementations at the same
     * time. One such example is the Camera 2.0 API.
     *
     * This field is interpreted by the module user and is ignored by the
     * HAL interface itself.
     */
    uint32_t version;

    /** reference to the module this device belongs to */
    struct hw_module_t* module;

    /** padding reserved for future use */
#ifdef __LP64__
    uint64_t reserved[12];
#else
    uint32_t reserved[12];
#endif

    /** Close this device */
    int (*close)(struct hw_device_t* device);

} hw_device_t;

typedef struct hw_module_methods_t {
    /** Open a specific device */
    int (*open)(const struct hw_module_t* module, const char* id,
            struct hw_device_t** device);

} hw_module_methods_t;

struct audio_module {
    struct hw_module_t common;
};

/**
 * Name of the hal_module_info
 */
#define HAL_MODULE_INFO_SYM         HMI

/**
 * Name of the hal_module_info as a string
 */
#define HAL_MODULE_INFO_SYM_AS_STR  "HMI"

struct audio_module HAL_MODULE_INFO_SYM = {
    .common = {
        .tag = HARDWARE_MODULE_TAG,
        .module_api_version = AUDIO_MODULE_API_VERSION_0_1,
        .hal_api_version = HARDWARE_HAL_API_VERSION,
        .id = AUDIO_HARDWARE_MODULE_ID,
        .name = "Generic audio HW HAL",
        .author = "The Android Open Source Project",
        .methods = &hal_module_methods,
    },
};```

**但是我们为什么要在每个硬件模块中都定义一个变量名为 HAL_MODULE_INFO_SYM 的变量呢？**

原因是**为了统一标准的接口**。这个硬件模块类型的变量主要是在 HAL 动态链接库加载时用到，它使得上层的 Framework 层打开所有的 HAL 动态链接库时都能找到名为 HAL\_MODULE\_INFO\_SYM（也就是 hmi）的硬件模块类型变量，然后通过这个变量再来打开它所管理的硬件设备，从而与之进行交互。这么做的好处就是 Framework 层中只需要用相同的 API，就能处理各个厂商所提供的不同的 HAL 动态链接库。

### step 3：定义 struct audio\_hw\_device 硬件设备结构体
每个硬件设备都需要通过一个结构体来表示，并且这个结构体的第一个成员必须是 hw\_device\_t 类型。而有关的对底层硬件设备的有关操作的函数指针接口，也是在这个结构体中定义。 struct audio\_hw\_device 硬件设备结构体的定义如下所示：
代码路径：`/hardware/libhardware/include/hardware/audio.h`

```c
struct audio_hw_device {
    /**
     * Common methods of the audio device.  This *must* be the first member of audio_hw_device
     * as users of this structure will cast a hw_device_t to audio_hw_device pointer in contexts
     * where it's known the hw_device_t references an audio_hw_device.
     */
    struct hw_device_t common;

    ......

    /* Returns audio input buffer size according to parameters passed or
     * 0 if one of the parameters is not supported.
     * See also get_buffer_size which is for a particular stream.
     */
    size_t (*get_input_buffer_size)(const struct audio_hw_device *dev,
                                    const struct audio_config *config);

    /** This method creates and opens the audio hardware output stream.
     * The "address" parameter qualifies the "devices" audio device type if needed.
     * The format format depends on the device type:
     * - Bluetooth devices use the MAC address of the device in the form "00:11:22:AA:BB:CC"
     * - USB devices use the ALSA card and device numbers in the form  "card=X;device=Y"
     * - Other devices may use a number or any other string.
     */

    int (*open_output_stream)(struct audio_hw_device *dev,
                              audio_io_handle_t handle,
                              audio_devices_t devices,
                              audio_output_flags_t flags,
                              struct audio_config *config,
                              struct audio_stream_out **stream_out,
                              const char *address);

    void (*close_output_stream)(struct audio_hw_device *dev,
                                struct audio_stream_out* stream_out);

    /** This method creates and opens the audio hardware input stream */
    int (*open_input_stream)(struct audio_hw_device *dev,
                             audio_io_handle_t handle,
                             audio_devices_t devices,
                             struct audio_config *config,
                             struct audio_stream_in **stream_in,
                             audio_input_flags_t flags,
                             const char *address,
                             audio_source_t source);

    void (*close_input_stream)(struct audio_hw_device *dev,
                               struct audio_stream_in *stream_in);

    ......

};

static struct hw_module_methods_t hal_module_methods = {
    .open = adev_open,
};

static int adev_open(const hw_module_t* module, const char* name,
                     hw_device_t** device)
{
     struct generic_audio_device *adev;
    int fd;

    if (strcmp(name, AUDIO_HARDWARE_INTERFACE) != 0)
        return -EINVAL;

    fd = open(AUDIO_DEVICE_NAME, O_RDWR);
    if (fd < 0)
        return -ENOSYS;

    adev = calloc(1, sizeof(struct generic_audio_device));

    adev->fd = fd;

    adev->device.common.tag = HARDWARE_DEVICE_TAG;
    adev->device.common.version = AUDIO_DEVICE_API_VERSION_2_0;
    adev->device.common.module = (struct hw_module_t *) module;
    adev->device.common.close = adev_close;

    adev->device.init_check = adev_init_check;
    adev->device.set_voice_volume = adev_set_voice_volume;
    adev->device.set_master_volume = adev_set_master_volume;
    adev->device.get_master_volume = adev_get_master_volume;
    adev->device.set_master_mute = adev_set_master_mute;
    adev->device.get_master_mute = adev_get_master_mute;
    adev->device.set_mode = adev_set_mode;
    adev->device.set_mic_mute = adev_set_mic_mute;
    adev->device.get_mic_mute = adev_get_mic_mute;
    adev->device.set_parameters = adev_set_parameters;
    adev->device.get_parameters = adev_get_parameters;
    adev->device.get_input_buffer_size = adev_get_input_buffer_size;
    adev->device.open_output_stream = adev_open_output_stream;
    adev->device.close_output_stream = adev_close_output_stream;
    adev->device.open_input_stream = adev_open_input_stream;
    adev->device.close_input_stream = adev_close_input_stream;
    adev->device.dump = adev_dump;

    *device = &adev->device.common;

    return 0;
}

struct generic_audio_device {
    struct audio_hw_device device;
    pthread_mutex_t lock;
    struct audio_stream_out *output;
    struct audio_stream_in *input;
    int fd;
    bool mic_mute;
};